Orthogonal frequency scan scheme in touch system

ABSTRACT

System and processes for transmitting orthogonal frequencies on a touch sensor are provided. In one example process, the rows of the sensor panel can have signals transmitted thereon having orthogonal frequencies. The orthogonal frequencies can be separated by a frequency spacing (Δf) that is at least the reciprocal of a measurement period τ (e.g., an integration time) of the touch sensor. Touch events cause and correspond to signals on the columns, which can be received by a receive system including appropriate amplifiers. The orthogonal frequencies can be detected by the receiver with a Fourier Transform or filter bank. Separate digitization and signal processing can be implemented for every column. The receiver can measure the quantity of each of the orthogonal transmitted signals present on each column, identifying the rows in touch with each column and may also provide additional (e.g., qualitative) information concerning the touch.

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has notobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice files or records, but otherwise reserves all copyright rightswhatsoever.

FIELD

The present invention relates in general to the field of user input, andin particular to user input systems which deliver a low-latency userexperience.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of thedisclosure will be apparent from the following more particulardescription of embodiments as illustrated in the accompanying drawings,in which reference characters refer to the same parts throughout thevarious views. The drawings are not necessarily to scale, emphasisinstead being placed upon illustrating principles of the disclosedembodiments.

FIG. 1 provides a high level block diagram illustrating an embodiment ofa low-latency touch sensor device.

FIG. 2 illustrates an embodiment of a layout for crossing conductivepaths that can be used in an embodiment of a low-latency touch sensordevice.

DETAILED DESCRIPTION

Disclosed are a sensor and method that provide detection of touch eventsfrom human fingers on a two-dimensional manifold and have the capabilityfor multiple simultaneous touch events to be detected and distinguishedfrom each other. In accordance with an embodiment, the touch events maybe detected, processed and supplied to downstream computationalprocesses with very low latency, e.g., on the order of one millisecondor less.

In an embodiment, the invention provides a projected capacitive methodthat has been enhanced for high update rate and low latency measurementsof touch events. The technique can use parallel hardware and higherfrequency waveforms to gain the above advantages. Also disclosed aremethods to make sensitive and robust measurements, which methods may beused on transparent display surfaces and which may permit economicalmanufacturing of products which employ the technique.

FIG. 1 illustrates certain principles of the invention in accordancewith an embodiment of a touch sensor 100. At reference no. 200, adifferent signal is transmitted into each of the surface's rows. Thesignals are designed to be “orthogonal”, i.e. separable anddistinguishable from each other. At reference no. 300, a receiver isattached to each column. The receiver is designed to receive any of thetransmitted signals, or an arbitrary combination of them, and toindividually measure the quantity of each of the orthogonal transmittedsignals present on that column. The touch surface 400 of the sensorcomprises a series of rows and columns (not all shown), along which theorthogonal signals can propagate. The rows and columns are designed sothat, when they are not being touched, a negligible amount of signal iscoupled between them.

With continued reference to FIG. 1, generally, the capacitive result ofa touch event in the proximity of both a row and column will cause anon-negligible amount of signal present on the row to be coupled to thecolumn, thus, touch events generally cause, and thus correspond to, thereceived signals on the columns. Because the signals on the rows areorthogonal, multiple row signals can be coupled to a column anddistinguished by the receiver. Likewise, the signals on each row can becoupled to multiple columns. For each column coupled to a given row, thesignals found on the column indicate which rows are being touchedsimultaneously with that column. The quantity of each signal received isgenerally related to the amount of coupling between the column and therow carrying the corresponding signal, and thus, may indicate the areaof the surface covered by the touch, the pressure of the touch, etc.

When a row and column are touched simultaneously, some of the signalthat is present on the row is coupled into the corresponding column. (Asused herein, the term touch or touched does not require actual physicalcontact, but rather, close proximity. Indeed, in many embodiments,physical contact is unlikely as there is generally a protective barrierbetween the rows and/or columns and the finger or other object of touch.Moreover, generally, the rows and columns themselves are not in touchwith each other, but rather, placed in a proximity that prevents morethan a negligible amount of signal to be coupled there-between.Generally, the row-column coupling results not from actual contactbetween them, nor by actual contact from the finger or other object oftouch, but rather, by the capacitive effect of bringing the finger (orother object) into close proximity—which close proximity resulting incapacitive effect is referred to herein as touch.)

The nature of the rows and columns is arbitrary and the particularorientation is irrelevant. Indeed, the terms row and column are notintended to refer to a square grid, but rather to a set of conductorsupon which signal is transmitted (rows) and a set of conductors ontowhich signal may be coupled (columns). In fact, it is not even necessarythat the rows and columns be in a grid at all. Other shapes are possibleas long as a touch event will simultaneously touch part of a “row” andpart of a “column”, and cause some form of coupling. For example, the“rows” could be in concentric circles and the “columns” could be spokesradiating out from the center. Moreover, it is not necessary for thereto be only two types signal propagation channels: instead of rows andcolumns, channels “A”, “B” and “C” may be provided, where signalstransmitted on “A” could be received on “B” and “C” or signalstransmitted on “A” and “B” could be received on “C”. It is also possiblethat the signal propagation channels can alternate function, sometimessupporting transmitters and sometimes supporting receivers. Manyalternative embodiments are possible and will be apparent to a person ofskill in the art after considering this disclosure.

As noted above, in an embodiment the touch surface 400 comprises of aseries of rows and columns, along which signals can propagate. Asdiscussed above, the rows and columns are designed so that, when theyare not being touched, a negligible amount of signal is coupled betweenthem. Moreover, a different signal is transmitted into each of the rows.In an embodiment, each of these different signals are orthogonal (i.e.separable and distinguishable) from one another. When a row and columnare touched simultaneously, a non-negligible amount of the signal thatis present on the row is coupled into the corresponding column. Thequantity of the signal that is coupled onto a column may be related tothe pressure or area of touch.

A receiver 300 is attached to each column. The receiver is designed toreceive non-negligible amounts any of the orthogonal signals, or anarbitrary combination of the orthogonal signals, and to identify thecolumns providing non-negligible amounts of signal. In an embodiment,the receiver may measure the quantity of each of the orthogonaltransmitted signals present on that column. In this manner, in additionto identifying the rows in touch with each column, the receiver canprovide additional (e.g., qualitative) information concerning the touch.In general, touch events may correspond to the received signals on thecolumns. For each column, the different signals received thereonindicate which of the corresponding rows is being touched simultaneouslywith that column. In an embodiment, the non-negligible quantity of eachsignal received may be related to the amount of coupling between thecorresponding row and column and may indicate the area of the surfacecovered by the touch, the pressure of the touch, etc.

Simple Sinusoid Embodiment

In this simple embodiment of the technology, the orthogonal signalsbeing transmitted into the rows are unmodulated sinusoids, each of whichhas a different frequency. In an embodiment, the frequencies are chosenso that they can be easily distinguished from each other in thereceiver. One way to select frequencies that they can be easilydistinguished from each other in the receiver involves providingsufficient spacing between them, and ensuring that no simple harmonicrelationships exist between them, thus mitigating non-linear artifactsthat may cause one signal to mimic another.

A “comb” of frequencies, where the spacing between adjacent frequenciesis constant, and the highest frequency is less than twice the lowest,will generally meet these criteria if the spacing between frequencies,Δf, is at least the reciprocal of the measurement period τ. For example,if it is desired to measure a combination of signals (from a column, forexample) to determine which row signals are present once permillisecond, then the frequency spacing must be greater than onekilohertz Δf>1/τ). According to this calculation, for a trivial casewith only ten rows, one could use the following frequencies:

Row 1: 5.000 MHz Row 2: 5.001 MHz Row 3: 5.002 MHz Row 4: 5.003 MHz Row5: 5.004 MHz Row 6: 5.005 MHz Row 7: 5.006 MHz Row 8: 5.007 MHz Row 9:5.008 MHz Row 10: 5.009 MHz

In an embodiment, frequency spacing should be substantially greater thanthis minimum to permit robust design. As a notional example, a 20 cm by20 cm touch surface with 0.5 cm row/column spacing would require fortyrows and forty columns and necessitate sinusoids at forty differentfrequencies. While a once per millisecond analysis rate would requireonly 1 KHz spacing, in an embodiment, an arbitrarily larger spacing isutilized for a more robust implementation. The arbitrarily largerspacing is subject to the constraint that the maximum frequency shouldnot be more than twice the lowest (i.e. f_(max)<2(f_(min))). In thisnotional example, a frequency spacing of 100 kHz with the lowestfrequency set at 5 MHz may be used, yielding a frequency list of 5.0MHz, 5.1 MHz, 5.2 MHz, etc. up to 8.9 MHz.

In an embodiment, each of the sinusoids on the list may be generated bya signal generator and transmitted on a separate row by the transmitter.To identify the rows and columns that are being simultaneously touched,a receiver receives any signals present on the columns and a signalprocessor analyzes the signal to determine which, if any, frequencies onthe list appear. In an embodiment, the identification can be supportedwith a frequency analysis technique (e.g., Fourier transform), or byusing a filter bank.

In an embodiment, from each column's signal, the receiver can determinethe strength of each frequency from the list of frequencies found in thesignal on that column. In an embodiment, where the strength of afrequency is greater than some threshold, the signal processoridentifies there being a touch event between the column and the rowcorresponding to that frequency. In an embodiment, signal strengthinformation, which may correspond to various physical phenomenaincluding the size of the touch tool, the pressure with which the toolis pressing down, the fraction of row/column intersection that is beingtouched, etc. may be used as an aid to localize the area of the touchevent.

In an embodiment, once the signals strengths have been calculated foreach frequency (corresponding to a row) for each column, atwo-dimensional map can be created, with the signal strength being thevalue of the map at that row/column intersection. In an embodiment, dueto physical differences in the touch surface at different frequencies,it will likely be necessary to normalize the signal strength responsesfor a given touch.

In an embodiment, the two-dimensional map data may be thresholded tobetter identify, determine or isolate touch events. In an embodiment,the two-dimensional map data may be used to infer information about theshape, orientation, etc. of the object touching the surface.

Of course, a sinusoid is not the only orthogonal signal that can be usedin the configuration described above, and indeed, as discussed above,any set of signals that can be distinguished from each other would work.Nonetheless, sinusoids have some advantageous properties that may permitsimpler engineering and more cost efficient manufacture of devices whichuse this technique. Sinusoids have a very narrow frequency profile (bydefinition), and need not extend down to low frequencies, near DC.Sinusoids are relatively unaffected by 1/f noise, which could be anissue for signals that extend to much lower frequencies.

In an embodiment, sinusoids may be detected by filter banks or frequencyanalysis techniques (e.g., Fourier transform), which can be implementedin a relatively efficient manner and which tend to have good dynamicrange characteristics, allowing them to detect and distinguish between alarge number of simultaneous sinusoids. In broad signal processingterms, the receiver's decoding of multiple sinusoids may be thought ofas a form of frequency-division multiplexing. In an embodiment, othermodulation techniques such as time-division and code-divisionmultiplexing could also be used, but these techniques may havedisadvantages when applied to an implementation of the present sensor.Time division multiplexing has good dynamic range characteristics, butrequires that a finite time be expended transmitting into (or analyzingreceived signals from) the touch surface, conflicting with the goal of alow latency device. Code division multiplexing has the same simultaneousnature as frequency-division multiplexing, but may encounter dynamicrange problems and may not distinguish as easily between multiplesimultaneous signals.

Modulated Sinusoid Embodiment

In an embodiment, a modulated sinusoid may be used in lieu of, and as anenhancement of, the simple sinusoid embodiment described above. The useof unmodulated sinusoids may cause radiofrequency interference to otherdevices near the touch surface, and thus, a device employing such mightencounter problems passing regulatory testing (e.g., FCC, CE). Inaddition, the use of unmodulated sinusoids may be susceptible tointerference from other sinusoids in the environment, whether fromdeliberate transmitters or from other interfering devices (perhaps evenanother identical touch surface). In an embodiment, such interferencemay cause false or degraded touch measurements in the described device.

In an embodiment, to avoid interference, the sinusoids may be modulatedor “stirred” prior to being transmitted by the transmitter in a mannerthat the signals can be demodulated (“unstirred”) once they reach thereceiver. Generally, any form of invertible transformation may be used,such that the transformation can be compensated for and the signalsrestored once they reach the receiver. Generally, it has been found thatsignals emitted or received using a modulation technique are lesscorrelated with other things, and thus, act more like mere noise, ratherthan appearing to be similar to, and/or being subject to interferencefrom, other signals present in the environment.

In an embodiment, the modulation technique utilized will cause thetransmitted data to appear fairly random, or least, unusual in theenvironment of the device operation. Two modulation schemes arediscussed below: Frequency Modulation and Direct Sequence SpreadSpectrum Modulation.

Frequency Modulation

Frequency modulation of the entire set of sinusoids keeps them fromappearing at the same frequencies by “smearing them out”. Becauseregulatory testing is generally concerned with fixed frequencies,transmitted sinusoids that are frequency modulated will appear at lowerlevels, and thus, be less likely to be a concern. Because the receiverwill “un-smear” any sinusoid input to it, in an equal and oppositefashion, the deliberately modulated, transmitted sinusoids can bedemodulated and will thereafter appear substantially as they did priorto modulation. Any fixed frequency sinusoids that enter (e.g.,interfere) from the environment, however, will be “smeared out” by the“unsmearing” operation. Accordingly, interference that might otherwisebe caused to the sensor is lessened by employing frequency modulation,e.g., to a comb of frequencies that, in an embodiment, are used in thetouch sensor.

In an embodiment, the entire set of sinusoids may be frequency modulatedby generating them all from a single reference frequency that is,itself, modulated. For example, a set of sinusoids with 100 kHz spacingcan be generated by multiplying the same 100 kHz reference frequency bydifferent integers. In an embodiment this technique can be accomplishedusing phase-locked loops. To generate the first 5.0 MHz sinusoid, onecould multiply the reference by 50, to generate the 5.1 MHz sinusoid,one could multiply the reference by 51, and so forth. The receiver canuse the same modulated reference to perform the detection anddemodulation functions.

Direct Sequence Spread Spectrum Modulation

In an embodiment, the sinusoids may be modulated by periodicallyinverting them on a pseudo-random (or even truly random) schedule knownto both the transmitter and receiver. Thus, in an embodiment, beforeeach sinusoid is transmitted to its corresponding row, it is passedthrough a selectable inverter circuit, the output of which is the inputsignal multiplied by +1 or −1 depending on the state of an “invertselection” input. In an embodiment, all of these “invert selection”inputs are driven from the same signal, so that the sinusoids for eachrow are all multiplied by either +1 or −1 at the same time. In anembodiment, the signal that drives the “invert selection” input may be apseudorandom function that is independent of any signals or functionsthat might be present in the environment. The pseudorandom inversion ofthe sinusoids spreads them out in frequency, causing them to appear likerandom noise so that they interfere negligibly with any devices withwhich they might come in contact.

On the receiver side, the signals from the columns may be passed throughselectable inverter circuits that are driven by the same pseudorandomsignal as the ones on the rows. The result is that, even though thetransmitted signals have been spread in frequency, they are despreadbefore the receiver because they have been multiplied by either +1 or −1twice, leaving them unmodified. Applying direct sequence spread spectrummodulation may spread out any interfering signals present on the columnsso that they act only as noise and do not mimic any of the set ofintentional sinusoids.

In an embodiment, selectable inverters can be created from a smallnumber of simple components and/or can be implemented in transistors ina VLSI process.

Because many modulation techniques are independent of each other, in anembodiment, multiple modulation techniques could be employed at the sametime, e.g. frequency modulation and direct sequence spread spectrummodulation of the sinusoid set. Although potentially more complicated toimplement, such multiple modulated implementation may achieve betterinterference resistance.

Because it would be extremely rare to encounter a particular pseudorandom modulation in the environment, it is likely that the sensors ofthe invention would not require a truly random modulation schedule. Theone likely exception is when more than one touch surface with the sameimplementation is being touched by the same person. In such a case, itmay be possible for the surfaces to interfere with each other, even ifthey use very complicated pseudo random schedules. In an embodiment,care is taken to design pseudo random schedules that are unlikely toconflict. In an embodiment, some true randomness is introduced into themodulation schedule. In an embodiment, randomness is introduced byseeding the pseudo random generator from a truly random source andensuring that it has a sufficiently long output duration (before itrepeats). Such an embodiment makes it highly unlikely that two touchsurfaces will ever be using the same portion of the sequence at the sametime. In an embodiment, randomness is introduced by exclusive or'ing(XOR) the pseudo random sequence with a truly random sequence. The XORfunction combines the entropy of its inputs, so that the entropy of itsoutput is never less than either input.

A Low-Cost Implementation Embodiment

Touch surfaces using the previously described techniques may have arelatively high cost associated with generating and detecting sinusoidscompared to other methods. Below are discussed methods of generating anddetecting sinusoids that may be more cost-effective and/or be moresuitable for mass production.

Sinusoid Detection

In an embodiment, sinusoids may be detected in a receiver using acomplete radio receiver with a Fourier Transform detection scheme. Suchdetection may require digitizing a high-speed RF waveform and performingdigital signal processing thereupon. Separate digitization and signalprocessing may be implemented for every column of the surface, thiswould permit the signal processor to discover which of the row signalsare in touch with that column. In the above-noted example, having atouch surface with forty rows and forty columns, would require fortycopies of this signal chain. Today, digitization and digital signalprocessing are relatively expensive operations, in terms of hardware,cost, and power. It would be useful to utilize a more cost-effectivemethod of detecting sinusoids, especially one that could be easilyreplicated and requires very little power.

In an embodiment, sinusoids may be detected using a filter bank. Afilter bank comprises an array of bandpass filters that can take aninput signal and break it up into the frequency components associatedwith each filter. The Discrete Fourier Transform (DFT, of which the FFTis an efficient implementation) is a form of a filter bank withevenly-spaced bandpass filters that is commonly used for frequencyanalysis. DFTs may be implemented digitally, but the digitization stepis expensive. It is possible to implement a filter bank out ofindividual filters, such as passive LC (inductor and capacitor) or RCactive filters (involving). Inductors are notoriously difficult toimplement well on VLSI processes, and discrete inductors are large andexpensive, so it may not be cost effective to use inductors in thefilter bank.

At lower frequencies (about 10 MHz and below), it is possible to buildbanks of RC active filters on VLSI. These might perform well, but couldtake up a lot of die space and require more power than is desirable.

At higher frequencies, it is possible to build filter banks with surfaceacoustic wave (SAW) filter techniques. These allow nearly arbitrary FIRfilter geometries. However, they require piezoelectric materials, whichare more expensive than straight CMOS VLSI, and may not allow enoughsimultaneous taps to integrate sufficiently many filters into a singlepackage, thereby raising the manufacturing cost.

In an embodiment, sinusoids may be detected using an analog filter bankimplemented with switched capacitor techniques on standard CMOS VLSIprocesses that employs an FFT-like “butterfly” topology. The die arearequired for such an implementation is a function of the square of thenumber of channels, meaning that a 64-channel filter bank using the sametechnology would require only 1/256th of the die area of the1024-channel version. In an embodiment, the complete receive system forthe low-latency touch sensor is implemented on a plurality of VLSI dies,including an appropriate set of filter banks and the appropriateamplifiers, switches, energy detectors, etc. In an embodiment, thecomplete receive system for the low-latency touch sensor is implementedon a single VLSI die, including an appropriate set of filter banks andthe appropriate amplifiers, switches, energy detectors, etc. In anembodiment, the complete receive system for the low-latency touch sensoris implemented on a single VLSI die containing n instances of ann-channel filter bank, and leaving room for the appropriate amplifiers,switches, energy detectors, etc.

Sinusoid Generation

Generating the transmit signals (e.g., sinusoids) in a low-latency touchsensor is generally less complex than detection, principally becauseeach row requires the generation of a single signal while the columnreceivers have to detect and distinguish between many signals. In anembodiment, sinusoids can be generated with a series of phase-lockedloops (PLLs), each of which multiply a common reference frequency by adifferent multiple.

In an embodiment, the low-latency touch sensor design does not requirethat the transmitted sinusoids are of very high quality, but rather,accommodates transmitted sinusoids that have more phase noise, frequencyvariation (over time, temperature, etc.), harmonic distortion and otherimperfections than may usually be allowable or desirable in radiocircuits. In an embodiment, the large number of frequencies may begenerated by digital means and then employ a relatively coarseanalog-to-digital conversion process. As discussed above, in anembodiment, the generated row frequencies should have no simple harmonicrelationships with each other, any non-linearities in the describedgeneration process should not cause one signal in the set to “alias” ormimic another.

In an embodiment, a frequency comb may be generated by having a train ofnarrow pulses filtered by a filter bank, each filter in the bankoutputting the signals for transmission on a row. The frequency “comb”is produced by a filter bank that may be identical to a filter bank thatcan be used by the receiver. As an example, in an embodiment, a 10nanosecond pulse repeated at a rate of 100 kHz is passed into the filterbank that is designed to separate a comb of frequency componentsstarting at 5 MHz, and separated by 100 kHz. The pulse train as definedwould have frequency components from 100 kHz through the tens of MHz,and thus, would have a signal for every row in the transmitter. Thus, ifthe pulse train were passed through an identical filter bank to the onedescribed above to detect sinusoids in the received column signals, thenthe filter bank outputs will each contain a single sinusoid that can betransmitted onto a row.

Transparent Display Surface

It may be desirable that the touch surface be integrated with a computerdisplay so that a person can interact with computer-generated graphicsand imagery. While front projection can be used with opaque touchsurfaces and rear projection can be used with translucent ones, modernflat panel displays (LCD, plasma, OLED, etc.) generally require that thetouch surface be transparent. In an embodiment, the present technique'srows and columns, which allow signals to propagate along them, need tobe conductive to those signals. In an embodiment, the presenttechnique's rows and columns, which allow radio frequency signals topropagate along them, need to be electrically conductive.

If the rows and columns are insufficiently conductive, the resistanceper unit length along the row/column will combine with the capacitanceper unit length to form a low-pass filter: any high-frequency signalsapplied at one end will be substantially attenuated as they propagatealong the poor conductor.

Visually transparent conductors are commercially available (e.g.indium-tin-oxide or ITO), but the tradeoff between transparency andconductivity is problematic at the frequencies that may be desirable forsome embodiments of the low-latency touch sensor described herein: ifthe ITO were thick enough to support certain desirable frequencies overcertain lengths, it may be insufficiently transparent for someapplications. In an embodiment, the rows and/or columns may be formedentirely, or at least partially, from graphene and/or carbon nanotubes,which are both highly conductive and optically transparent.

In an embodiment, the rows and/or columns may be formed from one or morefine wires that block a negligible amount of the display behind them. Inan embodiment, the fine wires are too small to see, or at least toosmall to present a visual impediment when viewing a display behind it.In an embodiment, fine silver wires patterned onto transparent glass orplastic can be used to make up the rows and/or columns. Such fine wiresneed to have sufficient cross section to create a good conductor alongthe row/column, but it is desirable (for rear displays) that such wiresare small enough and diffuse enough to block as little of the underlyingdisplay as appropriate for the application. In an embodiment, the finewire size is selected on the basis of the pixels size and/or pitch ofthe underlying display.

As an example, the new Apple Retina displays comprises about 300 pixelsper inch, which yields a pixel size of about 80 microns on a side. In anembodiment, a 20 micron diameter silver wire 20 centimeters long (thelength of an iPad display), which has a resistance of about 10 ohms, isused as a row and/or column and/or as part of a row and/or column in alow-latency touch sensor as described herein. Such 20 micron diametersilver wire, however, if stretched across a retina display, may block upto 25% of an entire line of pixels. Accordingly, in an embodiment,multiple thinner diameter silver wires may be employed as a column orrow, which can maintain an appropriate resistance, and provideacceptable response with respect to radiofrequency skin depth issues.Such multiple thinner diameter silver wires can be laid in a patternthat are not straight, but rather, somewhat irregular. A random orirregular pattern of thinner wires is likely to be less visuallyintrusive. In an embodiment, a mesh of thin wires is used; the use of amesh will improve robustness, including against manufacturing flaws inpatterning. In an embodiment, single thinner diameter wires may beemployed as a column or row, provided that the thinner is sufficientlyconductive to maintain an appropriate level resistance, and acceptableresponse with respect to radiofrequency skin depth issues.

FIG. 2 illustrates an embodiment of a row/column touch surface that hasa diamond-shaped row/column mesh. This mesh pattern is designed toprovide maximal and equal surface area to the rows and columns whilepermitting minimal overlap between them.

A touch event with an area greater than one of the diamonds will coverat least part of a row and a column, which will permit some coupling ofa row signal into the overlapped column. In an embodiment, the diamondsare sized to be smaller than the touching implement (finger, stylus,etc.). In an embodiment, a 0.5 cm spacing between rows and columnsperforms well for human fingers.

In an embodiment a simple grid of wires is employed as the rows andcolumns. Such a grid would provide less surface area for the rows andcolumns, but can suffice for radio frequency signals, and provide asufficient non-negligible coupling which can be detected by a receiver.

In an embodiment, the “diamond patterns” for the rows and columns, asshown in FIG. 2, can be created by using a randomly connected mesh ofthin wires that fills the space of the indicated shapes, or by combiningwire mesh and an another transparent conductor such as ITO. In anembodiment, thin wires may be used for long stretches of conductivity,e.g., across the entire screen, and ITO may be used for local areas ofconductivity, such as the diamond-shaped areas.

An Optical Embodiment

While radiofrequency and electrical methods of implementing thedescribed fast multi-touch technique have been discussed above, othermedia can be employed as well. For example, the signals can be opticalsignals (i.e., light), having waveguides or other means for the rows andcolumns. In an embodiment, the light, used for the optical signals maybe in the visible region, the infrared and/or the ultraviolet.

In an embodiment, instead of electrically conductive rows and columnsthat carry radiofrequency signals, the rows and columns could compriseoptical waveguides, such as optical fiber, fed by one or more lightsources that generate orthogonal signals and are coupled to thewaveguides by an optical coupler. For example, a different distinctwavelength of light could be injected into each row fiber. When a humanfinger touches a row fiber, some of the light in it will leak (i.e.,couple) into the finger, due to frustrated total internal reflection.Light from the finger may then enter one of the column fibers, due tothe reciprocal process, and propagate to a detector at the end of thefiber.

In an embodiment, optical signals may be generated with LEDs ofdifferent wavelengths, or by using optical filters. In an embodiment,custom interference filters are employed. In an embodiment, thedifferent wavelengths of light present on the fiber columns can bedetected using optical filter banks. In an embodiment, such opticalfilter banks may be implemented using custom interference filters. In anembodiment, wavelengths of light outside the visible spectrum (e.g.,infrared and/or ultraviolet light) may be used to avoid adding extravisible light to the display.

In an embodiment, the row and column fibers may be woven together sothat a finger can touch them simultaneously. In an embodiment, the wovenconstruction may be made as visually transparent as needed to avoidobscuring the display.

Further details regarding touch sensors, methods and uses of theinvention will be apparent from the disclosure of co-pending U.S.Provisional Patent Application No. 61/710,256 filed Oct. 5, 2012, theentire disclosure of which, including the source code appendix, isincorporated herein by reference.

Various modifications and alterations to the invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention. It should be understood that the inventionis not intended to be unduly limited by the specific embodiments andexamples set forth herein, and that such embodiments and examples arepresented merely to illustrate the invention, with the scope of theinvention intended to be limited only by the claims attached hereto.Thus, while the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

1. A system comprising: signal generator operable to generate aplurality of signals, each approximating one of a plurality ofpredetermined frequencies, the plurality of signals differing from theplurality of predetermined frequencies by having at least one selectedfrom the set of: phase noise, frequency variation, harmonic distortionand other imperfections; each frequency of the plurality ofpredetermined frequencies being a different frequency; each frequency ofthe plurality of predetermined frequencies is spaced from each otherfrequency of the plurality of predetermined frequencies by a spacingthat is at least a reciprocal of a predetermined measurement period of atouch sensor; each frequency of the plurality of predeterminedfrequencies having no simple harmonic relationship with any otherfrequency of the plurality of predetermined frequencies; and theplurality of signals simultaneously driving a plurality of rows of thetouch sensor.
 2. The system of claim 1, wherein none of the plurality ofsignals mimic any of the plurality of predetermined frequencies that itis not itself an approximation of.
 3. The system of claim 1, wherein themanner in which the generated plurality of signals differ from thepredetermined frequencies comprises harmonic distortion.
 4. The systemof claim 1, wherein the manner in which the generated plurality ofsignals differ from the predetermined frequencies comprises frequencyvariation.
 5. The system of claim 1, wherein the manner in which thegenerated plurality of signals differ from the predetermined frequenciescomprises phase noise.
 6. A method comprising the steps of: selectingfrequencies that can be easily distinguished from each other in areceiver, the selected frequencies having a sufficient spacing, Δf,between them and having no simple harmonic relationships between them;generating signals approximating the selected frequencies; transmittingthe generated signals on a plurality of conductors in a sensor;receiving signals on at least one conductor in the sensor during atleast a portion of the transmitting step; and processing the receivedsignals to determine a strength for each of the frequencies, thestrength for each of the frequencies being representative of a couplingbetween conductors.
 7. The method of claim 6, wherein the sensor has ameasurement period, τ, and wherein the spacing Δf, is at least thereciprocal of the measurement period τ.
 8. The method of claim 6,wherein a difference between the generated signals and the selectedfrequencies comprises harmonic distortion.